Monitoring a condition of a subject, such as vital signs including heart rate and respiration rate, can require expensive equipment, such as an electrocardiogram (EKG), a ballistocardiograph (BCG), a piezoelectric film, or an array of sensors. In addition to being prohibitively expensive for many situations, both EKGs and BCGs can be too cumbersome for use outside of medical facilities. EKGs, for example, typically necessitate attaching electrodes to the bodies of subjects, while BCGs rely on large, heavy, and unaesthetic force-measuring platforms. Some systems can also monitor the condition of presence or absence of a subject, but cannot monitor changes in position of the subject or accurately predict when a subject's position should change to benefit the subject.